1. The Field Of The Invention
The present invention relates to cable having low crosstalk characteristics and in particular to multi-conductor cable, such as the twenty-five pair cable commonly used in balanced systems including telephone applications.
2. The Prior Art
A well known method to reduce crosstalk between pairs of conductors is to employ a twisted pair concept in which the individual pairs of conductors are loosely twisted about one another. This concept is carried over into multi-conductor cable such as the twenty-five pair cable used in telephone applications with the adjacent pairs in the cable having different twist lengths.
While this type of cable is effective in reducing the crosstalk in the cable it does have a number of significant disadvantages including the fact that it is generally difficult to make such a cable, since the manufacturing requires forming a number of pairs of twisted conductors and securing the pairs together by some means, such as a woven thread tying the pairs together as described in U.S. Pat. Nos. 1,941,121 and 3,627,903. More importantly it is quite difficult to terminate such a cable, even with sophisticated connectors such as those disclosed in U.S. Pat. Nos. 3,617,983; 3,760,335; 3,824,530; 3,808,582. The above-noted connectors are manufactured under the name Champ by AMP Incorporated of Harrisburg, Pennsylvania along with a line of tools for effecting the termination of multi-conductor cable with the connectors.
Even conventional multi-conductor cable, such as the standard twenty-five pair cable used by telephone systems and which does not have a twist built into the pairs of conductors, is often difficult to terminate. It takes even a skilled craftsman up to several minutes to make each termination, since the individual pairs of wires must be identified, separated, and inserted into the appropriate holding comb or contacts themselves. The obvious disadvantage to such a type of cable is the expensive labor involved in making the termination and the substantial impossibility of accomplishing such a termination by automated machinery.
A further disadvantage is that the currently used multi-conductor cable, of the type discussed above, is relatively bulky and must be run through partitions or through conduit. The bulk does not allow it to be installed in an inconspicuous manner in some desirable circumstances, such as being run under a carpet or along some other planar surface not provided with conduit. Such conditions can only be satisfied by the use of multi-conductor flat flexible cable.
One flat flexible cable which approaches the solution of the above problem is described in U.S. Pat. No. 3,764,727. The cable described therein is known as a pseudo-twisted pair cable with each of a pair of conductors being formed on opposite sides of a single substrate. Each of the pairs follows a prescribed meandering pattern so that effectively the pairs of conductors have a plurality of crossing points which, to a certain extent, simulates the capacitive effect obtained by a true twisted pair. While this cable is substantially less difficult to terminate than the previously described multi-conductor cables, it is still quite expensive to manufacture and the electrical characteristics of such a cable are not as good as had been expected. It should also be noted that this last named patent teaches that simply stacking cables will not effect a crosstalk decrease. Rather there must be an offset stacking of cables according to a prescribed formula in order to reduce crosstalk.
Another flat flexible cable which has been conceived includes separate layers of conductors held together by an outer jacket which extends across such layers to hold such layers together. One such construction is shown in U.S. Pat. No. 3,757,029. There is a problem with use of this kind of construction, however, in that cables having twenty-five or more conductors in each layer are sufficiently wide that outer jacket coverings cannot hold the conductors of the cable layers together toward the center of the cable. The layers tend to part and shift relative position as the cable is flexed in use. This precludes precision placement of conductors in use necessary to maintain the kind of crosstalk reduction achieved by controlled placement of conductor pairs called for in the present invention.